[76.02] The Polarization of Reprocessed Annihilation Line Radiation in Compact Objects

M. G. Baring (Rice University)

Compact objects in the Milky Way provide excellent
candidates for probing source geometry using X-ray and soft
gamma-ray polarimetry, since they are luminous and possess
highly anisotropic environs. The Einstein source
1E1740.7-2942 near the Galactic Center (GC) is an
interesting case: it has been known to exhibit 511 keV line
emission that is somewhat variable. Improved observations of
hard X-rays just over a decade ago saw evidence of a bump at
around 170 keV that was not connected to positronium decay
emission. This bump was interpreted as part of a Compton
reflection feature of the 511 keV line that manifests itself
at 170-511 keV. The reflection is presumed to take place off
an accretion disk or some blob of material. The asymmetry of
such "target" geometries automatically implies significant
hard X-ray/soft gamma-ray polarization that couples to the
spectral shape. This paper computes such polarization
signatures for certain matter geometries and observational
perspectives, thereby providing environmental diagnostics
for 1E1740.7-2942 and other accreting compact objects such
as Nova Muscae. These model developments are germane to the
INTEGRAL hard X-ray/gamma-ray experiment, launched in late
2002, which can potentially measure polarization in this
energy band, sensitive to around the 10% level in
1E1740.7-2942 and other galactic and extragalactic sources.

This work was sponsored by NASA as part of the INTEGRAL
Guest Investigator Program.